In recent years, in the field of radio communication, MIMO communication technologies have been proposed in an attempt to improve communication performances, e.g., in an attempt to enhance transmission rate, increase communication capability, and improve reception qualities.
In a MIMO communication technology, a plurality of antennas are provided at both a radio communication apparatus on the transmission side and a radio communication apparatus on the reception side. The radio communication apparatus on the transmission side divides transmitted data into a plurality of data streams and simultaneously transmits the plurality of divided data streams at the same frequency band using a plurality of antennas. Meanwhile, using a plurality of antennas, the radio communication apparatus on the reception side receives a signal that is obtained by mixing, on a propagation path, the plurality of data streams transmitted from the radio communication apparatus on the transmission side. Using a channel matrix that represents the state of communication paths (channels), the radio communication apparatus on the reception side separates into individual streams the plurality of data streams transmitted from the radio communication apparatus on the transmission side from the signal received by each of the plurality of antennas, thereby restoring the transmitted data.
In radio communication systems using a MIMO technology, increasing the number of channels by increasing the number of antennas used to transmit or receive data may improve communication performance. Accordingly, attention is paid to increasing the number of antennas provided at a radio communication apparatus.
However, increasing the number of used antennas also increases the number of circuits such as amplifiers connected to the antennas, so power consumption of the radio communication apparatus will increase. For a radio communication apparatus on the reception side, a calculation process for separating a plurality of data streams individually from signals received by a plurality of antennas becomes complicated, and hence a complexity of the entire system will increase.
Accordingly, in order to solve the aforementioned problem associated with an increase in the number of used antennas, a technology has been proposed for selecting an antenna to be used for communication from a plurality of antennas provided at a radio communication apparatus.
The following prior art relates to a radio communication system that relies on a MIMO radio technology. That is, an encoded spatial data stream for transmission is delivered to a plurality of transmission chains by using a spatial diffusion matrix. The spatial diffusion matrix is used for a calculation to select an antenna of a transmitter. A receiver is aware of and may use a spatial diffusion matrix for calculations to select a transmitting antenna, select a receiving antenna, and select a joint transmitting/receiving antenna. The number of spatial data streams is smaller than the number of transmission chains or reception chains between a transmitter and a receiver, and the number of transmission chains or reception chains is smaller than the number of corresponding transmitting antennas or receiving antennas.
The following is another prior art. In the following descriptions, an antenna selecting method in accordance with the following prior art will be referred to as a “first prior example” for convenience. That is, to perform MIMO communication by selecting L (L is an integer that is two or greater) antennas from N (N is an integer that is two or greater) antennas provided at a radio communication apparatus, firstly, a first antenna is selected from the N antennas. An inverse matrix B1 of a channel matrix H1 of the selected first antenna is calculated. Next, the selected first antenna is combined with each of the not-selected N−1 antennas so as to generate N−1 antenna groups. An inverse matrix B2 of each of the N−1 antenna groups is calculated using the channel matrix H1 and the inverse matrix B1. A second antenna that conforms to a predetermined antenna selection criterion is selected from the not-selected N−1 antennas by using the calculated N−1 inverse matrixes B2. The predetermined antenna selection criterion includes a criterion of maximizing the minimum value of a signal to interference noise ratio (SINR) and a criterion of minimizing a means square error (MSE). Such processes are repeated until L antennas are selected.
In the aforementioned antenna selecting technology, the following antenna selecting method may be used to select an antenna with an outstanding communication performance from a plurality of antennas provided at a radio communication apparatus.
In the case of selecting L (L is an integer that is two or greater) antennas from N (N is an integer that is two or greater) antennas provided at a radio communication apparatus, there are NCL antenna combinations in total. Accordingly, all of the NCL antenna combinations are selected as antenna candidates for use in communication, and a calculation relating to communication performances for the NCL antenna combination candidates is performed. An antenna combination having the most outstanding communication performance is selected as antennas to be used for communication from the NCL antenna combinations. Such an antenna selecting method will hereinafter be referred to as a “second prior example” for convenience.
In accordance with the second prior example, an optimum antenna combination having the most outstanding communication performance may be selected. However, in the second prior example, when a radio communication apparatus is provided with many antennas, there will also be many antenna combination candidates, thereby increasing a complexity of a calculation process needed to select antennas to be used from the antenna combination candidates, with the result that an extraordinary amount of processing time is needed for the antenna selecting process.
Accordingly, an antenna selecting method needs to be considered that allows the complexity of a calculation process to be decreased and allows a processing time to be decreased even when a radio communication apparatus is provided with many antennas.
Making a communication performance obtained from a selected antenna combination completely different from a communication performance obtained from an optimum antenna combination as a result of enhancing an efficiency of an antenna selecting process in an attempt to limit a processing time and the complexity of a calculation process, eliminates the purpose of increasing the number of antennas to enhance the communication performance.
Accordingly, an antenna selecting method needs to be considered that, while limiting a processing time and the complexity of a calculation process, enables an optimum antenna combination or an antenna combination which achieves a communication performance that approximates a communication performance achieved by the optimum antenna combination to be precisely obtained.